Instrument and method for in situ rod adjustment

ABSTRACT

The instruments and methods of the present disclosure include an outer shaft, an inner shaft, a roller, and an actuator. In some examples, the outer shaft includes a lumen extending from a proximal end to a distal end. The inner shaft is disposed within the lumen. The roller rotatably is coupled with the distal end at an axis and configured to engage the fixation rod. The actuator operably couples with the inner shaft to translate the inner shaft within the lumen and cause rotation of the roller to impart linear movement on the fixation rod.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationSer. No. 61/722,993 entitled “Rod Pusher and Methods of Use” which wasfiled on Nov. 6, 2012 and is incorporated herein by reference in itsentirety.

FIELD

The present disclosure relates generally to spinal orthopedics, and moreparticularly to systems and methods for adjusting a spinal fixation rodpercutaneously and in situ.

BACKGROUND

The spine is a flexible column formed of a plurality of bones calledvertebrae. The vertebrae are hollow and piled one upon the other,forming a strong hollow column for support of the cranium and trunk. Thehollow core of the spine houses and protects the nerves of the spinalcord. The different vertebrae are connected to one another by means ofarticular processes and intervertebral, fibrocartilaginous bodies.Various spinal disorders may cause the spine to become misaligned,curved, and/or twisted or result in fractured and/or compressedvertebrae. It is often necessary to surgically correct these spinaldisorders.

The spine includes seven cervical (neck) vertebrae, twelve thoracic(chest) vertebrae, five lumbar (lower back) vertebrae, and the fusedvertebrae in the sacrum and coccyx that help to form the hip region.While the shapes of individual vertebrae differ among these regions,each is essentially a short hollow shaft containing the bundle of nervesknown as the spinal cord. Individual nerves, such as those carryingmessages to the arms or legs, enter and exit the spinal cord throughgaps between vertebrae.

The spinal disks act as shock absorbers, cushioning the spine, andpreventing individual bones from contacting each other. Disks also helpto hold the vertebrae together. The weight of the upper body istransferred through the spine to the hips and the legs. The spine isheld upright through the work of the back muscles, which are attached tothe vertebrae. While the normal spine has no side-to-side curve, it doeshave a series of front-to-back curves, giving it a gentle “S” shape. Ifthe proper shaping and/or curvature are not present due to scoliosis,neuromuscular disease, cerebral palsy, or other disorder, it may benecessary to straighten or adjust the spine into a proper curvature.

Generally the correct curvature is obtained by manipulating thevertebrae into their proper position and securing that position with arigid system of screws, rods, intervertebral spaces, and/or plates. Thevarious components of the system may be surgically inserted through openor minimally invasive surgeries. The components may also be insertedthrough various approaches to the spine including anterior, lateral, andposterior approaches and others in between.

Spinal fixation systems may be used in surgery to align, adjust, and/orfix portions of the spinal column, i.e., vertebrae, in a desired spatialrelationship relative to each other. Many spinal fixation systems employa spinal fixation rod for supporting the spine and for properlypositioning components of the spine for various treatment purposes.Vertebral anchors, comprising pins, bolts, screws, and hooks, engage thevertebrae and connect the fixation rod to different vertebrae. The size,length, and shape of the cylindrical rod depend on the size, number, andposition of the vertebrae to be held in a desired spatial relationshiprelative to each other by the apparatus.

During spinal surgery, a surgeon first exposes the spine posterior andattaches the vertebral anchors to selected vertebrae of the spine. Thesurgeon then inserts a properly shaped spinal rod into rod-receivingportions of the vertebral anchors to connect the selected vertebrae,thereby fixing the relative positions of the vertebrae. Generally, acontrolled mechanical force is required to bring together the spinal rodand a spinal implant, such as the vertebral anchors, in a convenientmanner. After insertion, a surgeon must insert a locking mechanism, suchas a set screw, into the vertebral anchor to lock the spinal rod to theimplant after the force for inserting the rod is removed.

In some surgeries, the surgeon may utilize a minimally invasive approachin which small incisions are made and through which the screws and rodsare inserted. Often the stabilization rods may inserted through one ofthe surgical entry points used to insert a screw. Accurate and preciseplacement of the stabilization rod in the receivers of the screws may bedifficult to achieve. Due to the minimally invasive approach, the rodmay require adjustment in situ. The present invention attempts to solvethese problems as well as others.

SUMMARY

Accordingly, the instruments and methods of the present disclosureinclude an outer shaft, an inner shaft, a roller, and an actuator. Insome examples, the outer shaft includes a lumen extending from aproximal end to a distal end. The inner shaft is disposed within thelumen. The roller rotatably is coupled with the distal end at an axisand configured to engage the fixation rod. The actuator operably coupleswith the inner shaft to translate the inner shaft within the lumen andcause rotation of the roller to impart linear movement on the fixationrod.

In other features, a connecting rod links a distal end of the innershaft with an off-axis point of the roller. A slot on the distal end ofthe outer shaft receives the axis of the roller and allows the roller totranslate distally and proximally from the fixation rod. A handlefixedly coupled with the proximal end of the outer shaft and a leverpivotally coupled with the handle shaft and linked to the inner shaft bya lever connector.

In still other features, the roller comprises a pair of rollers. Theroller includes a friction-enhancing surface. The roller includes aserrated surface.

In yet other features, the actuator is advanced to a drive position inwhich the inner shaft advances distally causing increased frictionbetween the roller and the fixation rod. A spring that returns theactuator to a rest position in which the inner shaft is drawnproximally. The actuator advances the inner shaft distally causing theroller to rotate and impart a force to translate the fixation rod.

The instrument may be used for advancing an in situ fixation rodrelative to an implanted screw assembly in a method comprising the stepsof inserting the instrument, positioning the actuator to impart linearmovement on the inner shaft to engage the roller with the fixation rodin a drive position, and positioning the actuator to impart linearmovement on the inner shaft to rotate the roller while engaged with thefixation rod.

In still other features, the method includes positioning the actuator toimpart linear movement on the inner shaft that increases frictionbetween the roller and the fixation rod and positioning the actuator toimpart linear movement on the inner shaft that disengages the rollerfrom the fixation rod.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an exemplary instrument for in situ rodadjustment according to the principles of the present disclosure.

FIG. 2A is a perspective view of a head portion for an exemplary screwassembly. FIG. 2B is a partial cross-sectional view of the screwassembly and a fixation rod.

FIG. 3A is a side view of the exemplary instrument of FIG. 1 engagedwith the screw assembly and fixation rod of FIG. 2B. FIG. 3B is a crosssectional view in the plane III-B of FIG. 3A.

FIGS. 4A-4D are a series of sequential cross-sectional views of thedistal end of the instrument engaged with the screw assembly andfixation rod and illustrating advancement of the fixation rod within thehead portion by actuation of the instrument according to the principlesof the present disclosure.

FIGS. 5A and 5B are perspective views of the distal end of theinstrument according to the principles of the present disclosure.

FIG. 6A is a side view of an inner face of a roller of the instrumentand FIG. 6B is side view of serrations of the roller according to theprinciples of the present disclosure.

FIGS. 7A and 7B are side views of an outer shaft of the instrumentaccording to the principles of the present disclosure. FIG. 7C is abottom view of the outer shaft of the instrument.

FIG. 8 is a cross-sectional view of a proximal end of the instrumentaccording to the principles of the present disclosure.

FIG. 9 is an exploded perspective view of the instrument according tothe principles of the present disclosure.

DETAILED DESCRIPTION

Embodiments of the invention will now be described with reference to theFigures, wherein like numerals reflect like elements throughout. Theterminology used in the description presented herein is not intended tobe interpreted in any limited or restrictive way, simply because it isbeing utilized in conjunction with detailed description of certainspecific embodiments of the invention. Furthermore, embodiments of theinvention may include several novel features, no single one of which issolely responsible for its desirable attributes or which is essential topracticing the invention described herein. The words proximal and distalare applied herein to denote specific ends of components of theinstrument described herein. A proximal end refers to the end of aninstrument nearer to an operator of the instrument when the instrumentis being used. A distal end refers to the end of a component furtherfrom the operator and extending towards the surgical area of a patientand/or the implant.

The foregoing and other features and advantages of the invention areapparent from the following detailed description of exemplaryembodiments, read in conjunction with the accompanying drawings. Thedetailed description and drawings are merely illustrative of theinvention rather than limiting, the scope of the invention being definedby the appended claims and equivalents thereof.

The present disclosure relates to instruments and methods for advancinga fixation rod in situ and percutaneously as in a minimally invasivespinal surgery. In some examples, the instrument may be referred to as arod pusher in that the instrument “pushes” the rod into position withinone or more rod-receiving members of a screw assembly such as a bodymember of a polyaxial screw assembly. Although the examples provideherein generally refer to polyaxial screws, fixed screws, monoaxialscrews, and uni-planar screws are all contemplated as they all generallyinclude a rod-receiving member on proximal ends of the screw assemblies.The rod pusher may operably couple to a screw assembly to move,translate, or displace the fixation rod. The rod pusher may generally beused to move, translate, or displace any instrument or device that maybe required to move along an axis.

As shown in FIG. 1, the rod pusher 100 generally comprises an outershaft 110 including a proximal end operably coupled to a handle 180 anda lever 190. The outer shaft 110 extends along a generally longitudinalaxis 104 of the rod pusher 100, wherein the outer shaft 110 includes aproximal end 112 and a distal end 114 generally along the longitudinalaxis 104. The longitudinal axis 104 is generally shown along the y-axis.The distal end 114 of the outer shaft 110 is operably coupled with atleast one roller 140, which operates to conform or couple with afixation rod, as further detailed below. The proximal end 112 of theouter shaft 110 is operably coupled with the handle 180 and the lever190 to rotate the at least one roller 140 and operably move the fixationrod coupled with or engaged with the roller 140. In one embodiment, theroller 140 operably moves the fixation rod substantially along thez-axis or cranial-caudal direction 106, alternatively the roller 140 mayoperably move the fixation rod substantially along the along the x-axisor y-axis. In one embodiment, the roller 140 operably engages thefixation rod and moves the fixation rod along a path substantiallyparallel to the z-axis (and perpendicular to the longitudinal axis 104)upon operation of the lever 190 and disengages with the fixation rodupon release of the lever 190.

Referring now to FIGS. 2A-2B, the rod pusher 100 is operably coupledwith a head portion 604 of a screw assembly 600, which may be configuredwith a threaded portion 202 of the screw assembly 600 or any other screwaccording to the prior art. For example, the head portion 604 may couplewith a rounded portion 214 on the proximal end of the threaded portion202 in a poly-axial screw configuration. A channel 606 running throughthe interior of the head portion 604 is configured to receive thefixation rod 300. A projection 612 protrudes from each of the recessedsurfaces 610. For example, the projection 612 may be a half-cylindershaped projection configured to engage. The distal end 114 of the outershaft 110 may operably couple or seat with the projections 612, such asto secure the roller 140 within the channel 606.

A width W1 is between the channel 606, which permit the roller to 140 tocouple with the fixation rod 300. An outer surface 608 of the headportion 604 includes two recessed surfaces 610 that are separated by thewidth W2. The outer surfaces of the projections 612 may be separated bya width W3 that is greater than the width W1. Once the fixation rod 300is seated in the channel 606, the rod pusher 100 may be engaged withinthe channel 606 and the roller 140 may be engaged with the fixation rod300. After the fixation rod 300 is appropriately moved or positioned, aset screw 210 may be attached to compress the fixation rod 300 androunded portion 214 of the threaded portion 202 together with the headportion 604. Alternatively, a bushing (not shown) may be disposedbetween the fixation rod 300 and the rounded portion 214 as known in theart.

As shown in FIGS. 3A-3B, the instrument 100 may be inserted into a screwextender 400 that is attached to the screw assembly 600. The screwextender 400 may be a generally elongated tube with various openingsthat enable access to the screw assembly 600 through a minimallyinvasive surgical approach. The extender 400 may include a passagewaythrough which the outer shaft 110 of the instrument 100 may be inserted.Examples of screw extender systems may be found in U.S. ApplicationPublication No. 2009/0171391 which is incorporated herein by referencein its entirety.

As shown in FIGS. 4A-4B, the distal end 114 of the outer shaft 110 isoperably coupled to the roller 140 and the roller 140 operably engagesthe fixation rod 300 when the fixation rod 300 is with the channel 606of the screw head assembly 200. A push rod 150 is coaxially coupledwithin the outer shaft 110; a distal end 152 of the push rod 150 isoperably coupled with a proximal end 162 of the connecting rod 160. Theroller 140 includes an axis 142 about which the roller 140 rotates,generally shown by arrow 141. The roller 140 may rotate clockwise orcounter-clockwise. Alternately, the roller 140 may rotate in only onedirection.

The connecting rod 160 is operably coupled to, engaged with, and/or incontact with a surface on the roller 140. A distal end 164 of theconnecting rod 160 is rotatably coupled to an off-axis point 166 on thesurface of the roller 140. The push rod 150 longitudinally moves alongthe longitudinal axis, shown as arrow 156, towards the distal end 114 ofthe outer shaft 110, which longitudinally moves the connecting rod 160to rotationally move the roller 140 by applying a torque due to theconnection at the off-axis point 166. The roller 140 is operably coupledwith at least one surface of the fixation rod 300, such that therotational movement of the roller 140 imparts force on the fixation rod300 along the z-axis or in the general cranial-caudal direction, asshown by arrow 301. For example, the roller 140 may impart a frictionalforce on the rod 300.

In one embodiment, the roller 140 is not engaged with the fixation rod300 until the push rod 150 distally displaces the connecting rod 160 asto push the roller 140 distally towards the fixation rod 300. In oneembodiment, the roller 140 may disengage with the fixation rod 300 bythe proximal movement of the connecting rod 160, which may proximallymove the roller 140 when the lever 190 is released. In one embodiment,the connecting rod 160 and/or the roller 140 may be operably coupledwith a spring as ensure pressure conformance with the fixation rod 300.In another embodiment, the roller 140 includes a plurality of serrations144 along the outer circumference of the roller 140 as to conform to thecontour of at least a portion of the surface of the fixation rod 300.

In one embodiment, the distance of the longitudinal displacement of thefixation rod 300 may be correlated to the rotations of the roller 140.For example, a larger roller 140 may be used to provide for a longer orlarger longitudinal displacement of the fixation rod 300. Additionally,the fixation rod 300 may be longitudinally displaced in set or discreteincrements, as determined by the roller 140 engaging, rotating, anddisengaging with the fixation rod 300. The longitudinal increments maybe determined by the operator, size of the roller, and the like. Forexample, half a rotation of the roller may correlate to a particular ordiscrete increment of longitudinal displacement of the fixation rod;alternatively, one-quarter of a rotation of the roller may correlate toanother discrete increment of longitudinal displacement of the fixationrod.

As shown in FIGS. 5A-5B, the distal end 114 of the outer shaft 110 mayinclude a first arm 120 a and a second arm 120 b that project distallyand operably couple the connecting rod 160 and the roller 140. The firstarm 120 a and second arm 120 b are reciprocal in nature and mirroredcomponents as described herein. The first arm 120 a and second arm 120 binclude a slotted feature 122 on the proximal portion and a curved slot124 on the distal portion of the first arm 120 a and second arm 120 b.The slotted feature 122 operably couples the proximal portion 162 of theconnecting rod 160 to permit the connecting rod 160 to move distally. Inone embodiment, the proximal portion 162 of the connecting rod 160 maybe operably coupled with the slotted feature by way of a pin. The curvedslot 124 operably couples the central axis 142 of the roller 140 topermit the roller 140 to move proximally and distally a small amount todisengage and engage the rod 300 and also rotate about the central axis142. As shown in FIG. 5B, the distal end 114 may include a first roller140 a and a second roller 140 b, whereby the serrations 144 face eachother, or the Angle A_(S) face each other, or towards the longitudinalaxis and the first roller 140 a and second roller 140 b are rotatablycoupled by way of corresponding central lips 146.

As shown in FIGS. 6A-6B, the roller 140 includes the plurality ofserrations 144 along the circumference of the roller 140 and around thecentral axis 142. In one embodiment, the roller 144 includes a centrallip 146 about to engage an adjacent roller 144 or to engage the distalend 114 of the outer shaft 110. The off-axis point 166 of engagement ofthe connecting rod 160 may be operably coupled to an off-axis opening148, through which a pin or other means may rotatably couple the roller140 to the connecting rod 160. The serrations 144 traverse at least aportion of the thickness T_(R) of the roller 140. Although serrations144 are shown, any other feature may be provided to increase frictionforces between the roller 140 and the rod 300 so as to enhance theability of the roller 140 to advance the rod 300. For example, varioussurface treatments may be used including roughening, coating, and/orrubberized layers may increase the friction forces.

The serrations 144 are set at an angle A_(S) off of the y-axis of theroller 140, such as to permit the serrations 144 to conform to thecontour of the surface of the fixation rod 300. The angle A_(S) may beset between 0-90 degrees depending on the degree of conformation andengagement of the serrations with the surface of the fixation rod. Inone embodiment, the angle A_(S) is at least 45 degrees, which maycorrespond to a conformance of at least ⅛ of the exterior surface of thefixation rod. Alternatively, the serrations 144 may by curved orgenerally C-shaped as to conform to at least ½ of the exterior surfaceof the fixation rod. Alternatively, the serrations 144 may include anangle A_(S) of between about 0-45 degrees for conformance of betweenabout 1/10 to ⅓ of the exterior surface of the fixation rod 300. Theangle A_(S) or curvature of the serrations 144 may be adjusted dependingon the degree of conformance of the serrations with the surface of thefixation rod.

As shown in FIGS. 7A-7B, the first and second arms 120 a and 120 bprotrude from the distal end 114 of the outer shaft 110. In oneembodiment, the first and second arms 120 a, 120 b form a roller opening126 to position the roller 140 therebetween and permit rotation of theroller 140 between the roller opening 126. The slotted feature 122 isslotted through the thickness of the first and second arms 120 a, 120 b,and the curved slot 124 traverses the thickness of the first and secondarms 120 a, 120 b. As shown in FIG. 7C, the first and second arms 120 a,120 b include a generally curved outer surface, which may becomplementary to at least a portion of the head portion 604. The outershaft 110 includes a central lumen 116 traversing from the proximal end112 to the distal end 114. The roller opening 126 generally includesthickness T_(O), which permits the placement of at least one or tworollers 140 therebetween. The outer shaft 110 includes a threadedsection 111 on the proximal end 112, as to be operably coupled to thehandle 180. In one embodiment, the proximal end 112 may include astaggered section for additional securement of the proximal end 112 tothe handle 180.

As shown in FIGS. 8 and 9, the distal end of the handle 180 is operablycoupled with the proximal end 112 of the outer shaft 110, with the pushrod 150 coaxially disposed through the lumen of the outer shaft 110 anda lumen 182 in the handle 180. The threaded end 111 of the outer shaft110 may threadably coupled with the distal end of the handle 180 andfurther secured to the distal end of the handle 180 by way of a lockingcap 184 and spring 185 coaxially disposed over the proximal end 112 ofthe outer shaft 110. The push rod 150 may include a threaded proximalportion 151 to operably engage with a translation nut 192 coaxiallydisposed within the lumen 182 of the handle 180. The translation nut 192is longitudinally displaced by a lever connector 198 that is operablyconnected to the lever 190.

When an operator squeezes the lever 190, the level connector 198longitudinally displaces the translation nut 192 towards the distal endof the handle 180, thereby displacing or translating the push rod 150connected within the handle lumen 182, and distally moving the connectorrod 160 to displace the roller 140 to engage the fixation rod, rotatethe roller 140 to move the fixation rod. When the handle 180 isreleased, the lever connector 198 moves the translation nut 190proximally within the handle lumen 182, thereby moving the push rod 150towards the proximal end and move the connector rod proximally towithdraw the roller 140 from the fixation rod. Optionally, a spring maybe coupled with the lever connector 198 in order to return the lever 190to its original position.

In one embodiment, the displacement of the lever 190 and translation nut192 may be correlated to the longitudinal displacement of the push rod150 and subsequent rotations of the roller 140. For example, if thelever 190 is displaced a larger distance, the push rod 150 may also bedisplaced a larger distance for a greater number of rotations of theroller 140 and longer longitudinal displacement of the fixation rod 300.Additionally, the lever 190 and the translation nut 192 may be displacedin set or discrete increments, as determined by a ratcheting or lockingmechanism. The displacement increments may be determined by theoperator, size of the ratchets, the locking distance, and the like. Forexample, the lever 190 may be only displaced towards the handle aparticular or discrete distance, which would correlate to a particularor discrete displacement of the translation nut 192 and the push rod150.

As can be understood by one skilled in the art, the rod pusher 100and/or any of its components may have any size, shape, length,thickness, height, weight, or any other parameters. Such parameters maybe selected by the surgeon (or other qualified professional) forperformance of specific procedures. Further, the rod pusher 100 and/orany of its components may be manufactured from metal, plastic, syntheticmaterial, or other suitable materials, or any combination thereof. Inone embodiment, the rod pusher 100 is composed of titanium, Nitinol, orstainless steel, or any medical grade polymer, such as Polyether etherketone (PEEK).

In some embodiments, various lengths and configurations may also includevarious features to accommodate different applications for the rodpusher. The rod pusher can be constructed of various materials to aid inradio translucency, strength, flexibility, and integration with anatomy,etc.

Example embodiments of the methods and systems of the present inventionhave been described herein. As noted elsewhere, these exampleembodiments have been described for illustrative purposes only, and arenot limiting. Other embodiments are possible and are covered by theinvention. Such embodiments will be apparent to persons skilled in therelevant art(s) based on the teachings contained herein. Thus, thebreadth and scope of the present invention should not be limited by anyof the above-described exemplary embodiments, but should be defined onlyin accordance with the following claims and their equivalents.

While the invention has been described in connection with variousembodiments, it will be understood that the invention is capable offurther modifications. This application is intended to cover anyvariations, uses, or adaptations of the invention following, in general,the principles of the invention, and including such departures from thepresent disclosure as, within the known and customary practice withinthe art to which the invention pertains.

The invention claimed is:
 1. An instrument for positioning a fixationrod, comprising: an outer shaft including a lumen extending from aproximal end to a distal end and an elongated slot on the distal end; aninner shaft within the lumen; a roller rotatably coupled at an axis ofrotation within the elongated slot on the distal end and configured toengage the fixation rod, the axis of rotation and the slot cooperatingto allow the roller to translate as a whole distally and proximally fromthe fixation rod by moving the axis of rotation distally and proximallywithin the slot; a connecting rod linking a distal end of the innershaft with an off-axis point of the roller, a pin coupling a proximalend of the connecting rod to the elongated slot; and an actuatoroperably coupled with the inner shaft to translate the inner shaftwithin the lumen and cause rotation of the roller to impart linearmovement on the fixation rod.
 2. The instrument of claim 1, wherein theactuator comprises a handle fixedly coupled with the proximal end of theouter shaft and a lever pivotally coupled with the handle and linked tothe inner shaft by a lever connector.
 3. The instrument of claim 1,wherein the roller comprises a pair of rollers.
 4. The instrument ofclaim 1, wherein the roller includes a friction-enhancing surface. 5.The instrument of claim 1, wherein the roller includes a serratedsurface.
 6. The instrument of claim 1, wherein the actuator is advancedto a drive position in which the inner shaft advances distally causingincreased friction between the roller and the fixation rod.
 7. Theinstrument of claim 1, further comprising a spring that returns theactuator to a rest position in which the inner shaft is drawnproximally.
 8. The instrument of claim 1, wherein the actuator advancesthe inner shaft distally causing the roller to rotate and impart a forceto translate the fixation rod.
 9. An instrument for positioning afixation rod, comprising: an outer shaft including a lumen extendingfrom a proximal end to a distal end, the distal end including anelongated slot and a curved slot; an inner shaft within the lumen; aroller rotatably coupled by a central axis extending through the curvedslot of the distal end and configured to engage the fixation rod,wherein the roller rotates about the central axis, wherein the curvedslot is curved to allow the roller to translate as a whole distally andproximally from the fixation rod by moving the central axis distally andproximally within the elongated slot; and a connecting rod linking adistal end of the inner shaft with an off-axis point of the roller, apin coupling a proximal end of the connecting rod to the elongated slot;and an actuator operably coupled with the inner shaft to translate theinner shaft within the lumen and cause rotation of the roller to impartlinear movement on the fixation rod.
 10. The instrument of claim 9,wherein the actuator comprises a handle fixedly coupled with theproximal end of the outer shaft and a lever pivotally coupled with thehandle and linked to the inner shaft by a lever connector.
 11. Theinstrument of claim 9, wherein the roller comprises a pair of rollerswith serrated surfaces to enhance friction between the rollers and thefixation rod.
 12. The instrument of claim 9, wherein the actuatoradvances the inner shaft distally causing the roller to rotate andimpart a force to translate the fixation rod.
 13. The instrument ofclaim 9, wherein the elongated slot allows the central axis of theroller to move proximally and distally relative to the distal end todisengage and engage the fixation rod.